I am not sure of your question. "a" or "an" is an article. You can learn more about articles here at[url="http://webster.commnet.edu/grammar/determiners/determiners.htm"]Articles, Determiners, and Quantifiers[/url].
"a / an" does seem a bit strange with 90,000, doesn't it?
There is no such thing as "a 90,000" or "a gangsters", of course!
What you have come across is an idiom usually used with large numbers.
The words "an estimated" are frequently used before a very large number or percentage to show that the number is not exact. It is not a mistake in which the singular article
There are an estimated 90000 gangsters in the country.
If your question pertains to the use of 'a' and 'an', I think the following might help you out. Use 'an' before all words beginning with the vowels: a, e, i, o u. This is done to facilitate pronunciation. For example, if we say 'a apple', it is a bit difficult to say; it is easier to say 'an apple'. Similarly, 'a elephan